PITTSBURGH, Pennsylvania (WABC) -- A gunman who's believed to have spewed anti-Semitic slurs and rhetoric on social media barged into a baby-naming ceremony at a Pittsburgh synagogue on Saturday and opened fire, killing 11 people in one of the deadliest attacks on Jews in U.S. history.

The 20-minute attack at Tree of Life Congregation in the Squirrel Hill neighborhood left at least six others wounded, including four police officers who dashed to the scene, authorities said.

The suspect, Robert Bowers, traded gunfire with police and was shot several times. Bowers is in fair condition at a hospital.

The U.S. Attorney for the Western District of Pennsylvania announced that Bowers is facing 29 charges:

-11 counts of Obstruction of Exercise of Religious Beliefs Resulting in Death -11 counts of Use of a Firearm to Commit Murder During and in Relation to a Crime of Violence -4 counts of Obstruction of Exercise of Religious Beliefs Resulting in Bodily Injury to a Public Safety Officer -3 counts of Use and Discharge of a Firearm During and in Relation to a Crime of Violence

"Please know that justice in this case will be swift and it will be severe," Scott Brady, the chief federal prosecutor in western Pennsylvania, said at a late-afternoon news conference, characterizing the slaughter as a "terrible and unspeakable act of hate."

Six people were injured, including four officers whose injuries are non-life-threatening. Three of those officers were shot.

The other injuries are "critical and serious in nature," said Wendell Hissrich, Pittsburgh's Public Safety director. The University of Pittsburgh Medical Center system said it's treating multiple victims.

The Tree of Life is in the vicinity of Shady Avenue and Wilkins Avenue in the Squirrel Hill neighborhood, a relatively affluent area about 10 minutes from downtown Pittsburgh.

Rabbi Alvin Berkun, the synagogue's rabbi emeritus, told ABC News he had heard that the suspect at one point was in his office on the third floor of the synagogue. He had stayed home from services because his wife was sick and that he has not heard from friends who were in the building.

Berkun said Tree of Life hosts three different religious groups that hold three services on Saturday mornings and guessed that there were approximately 75 people in the building at the time of the shooting.

President Donald Trump tweeted that he is watching the events unfold.

Watching the events unfolding in Pittsburgh, Pennsylvania. Law enforcement on the scene. People in Squirrel Hill area should remain sheltered. Looks like multiple fatalities. Beware of active shooter. God Bless All!

Before boarding Air Force One, Trump addressed the shooting in Pittsburgh and called the "hate in our country" a "terrible thing." He said the results would have been better if the synagogue had an armed guard and also called for strengthening the death penalty.

WTAE-TV reported that this synagogue often has security, but during the attack, it did not. The synagogue's security director is a former FBI agent.

Israeli Prime Minister Benjamin Netanyahu has condemned the attack on Pittsburgh's Tree of Life synagogue.

"I was heartbroken and appalled by the murderous attack on a Pittsburgh synagogue today," Netanyahu said in a video message posted on Twitter